BuddyPress.org

Find :
div#content #padder – increase margin-right to 300px

#sidebar change width to 300px and margin-left to -301px

However it looks as though you are using the default theme and changes like this should be made to a child theme in as much as you create a new theme directory and set up a new stylesheet that calls the default.css and other files but allows you to add override styles into this new styles.css.

Edit/ just noticed that your are actively editing / changing your theme layout so my earlier advice may or may not hold good